It is well known to fabricate apertured thermoplastic webs for a variety of applications, and the prior art is replete with disclosures relating to such webs and to processes for making them. However, most of the prior art apertured thermoplastic webs lack the desired appearance and hand to make them suitable for use as a facing sheet for sanitary products (e.g., sanitary napkins and incontinent pads), as well as for a variety of other applications wherein substantially cloth-like properties are desired and/or required.
In a substantial number of processes high density polyethylene, polypropylene, and other high modulus, crystalline polymers have been formed into plastic sheets, and then embossed with a variety of patterns designed to form thin areas in such sheets. These sheets, subsequent to embossing, are oriented either uniaxially or biaxially to cause the crystalline web to actually open up, or fracture, into a network of apertures in the thinned regions. Representative patents disclosing such a process are U.S. Pat. Nos. 3,914,365; 3,441,638; 3,488,415; 4,075,379; 3,922,329; 4,207,375; 4,186,781; 4,274,251; and 4,568,596.
In U.S. Pat. No. 3,386,876, Wyckoff discloses a process for manufacturing an apertured, thermoplastic web in which the apertures are defined by a plurality of uniaxially drawn strands which are separated from each other by undrawn portions. One approach to initially forming the apertures before stretching the film is to punch the apertures into the web in a repeating, hexagonal pattern.
As is shown in FIG. 10 of the Wyckoff '876 patent, the biaxially oriented web, which initially is formed with a hexagonal pattern of apertures, has a plurality of spoke-like, uniaxially drawn strands emanating from a central undrawn hub portion, and terminating at their opposite ends in similar undrawn portions. Wyckoff's structures are disclosed as being made from polypropylene, not from low density or ultra low density polyethylene or other low crystallinity polymers. As will be seen from the description of the present invention, the structure achieved in the Wyckoff patent is not desired in the present invention, and indeed would not be formed in the polymeric materials employed in the present invention.
In order to form apertured thermoplastic films or webs which closely simulate in appearance and hand fibrous non-woven and woven fabrics, applicants have recognized that the thermoplastic films should be extruded from softer (i.e., lower secant modulus) polymers than the prior art crystalline polymers employed in connection with the above discussed prior art processes. However, processing these softer, low crystallinity polymers to form apertured thermoplastic films simulating in appearance and hand the properties of conventional fibrous nonwoven and woven fabrics has not been an easily attainable objective.
In particular, applicants discovered that a number of embossing patterns and arrangements which successfully have been employed to form apertured thermoplastic webs from high density polyethylene and other high crystallinity polymers do not form a desirable apertured structure in lower crystallinity polymers. In particular, applicants discovered that thinned regions formed in soft, low crystallinity polymers do not tend to open up into apertures in a predictable manner, when the extruded polymer films are either uniaxially or biaxially oriented.
In U.S. Pat. No. 4,842,794, the present inventors disclose a unique, two-sided embossing arrangement which, unlike prior art, single-sided embossing arrangements, can be effectively utilized to form an apertured film from soft, non-crystalline polymers. In accordance with the teachings of '794 patent the two-sided embossing pattern actually formed slits through the web during melt embossing, and the film is subsequently biaxially stretched to open up the slit regions into a pattern of defined apertures. In other words, prior to the instant invention applicants determined that the specific two-sided embossing arrangement disclosed in the '794 patent functioned to actually slit the extruded polymer film, thereby making it possible to enlarge the slits into apertures by the subsequent step of biaxially orienting the film.
There is absolutely no suggestion in the '794 patent that single-sided embossing (e.g., pattern roll against smooth surface roll) could be employed to form an apertured web in polymers having a secant modulus less than 50,000 psi. In fact, applicants' prior experience with single-sided embossing arrangements, in connection with the processing of high density polyethylene webs, was that thinned (not slit) regions were formed, and that such thinned regions actually cracked, or fractured, to form a defined pattern of apertures when the extruded film was either uniaxially or biaxially oriented. However, prior to the invention forming the subject matter of the present application, applicants also had determined that thinned regions formed in soft, very low density polyethylene (e.g., Union Carbide DFDA 1137) by the use of some of the same embossing patterns employed to form thinned regions in high density polyethylene webs, did not crack and form apertures upon subsequent biaxial orientation on the processing equipment operated and owned by applicant's assignee.
Although an apertured fabric can be formed from extruded low density polyethylene employing the two-sided embossing arrangement disclosed in the Hovis et al. '794 patent, that product tended to have a number of strand-like polymer sections projecting in multiple planes, providing an undesired tactile feel to the product.
Thus, prior to the present invention, the prior art suggested that single-sided embossing patterns employed to form thin regions in extruded, crystalline polymers would form the same type of thin regions in softer, low-density, very low density and ultra low density polyethylene or other low crystallinity polymers. Moreover, although the thinned regions in the highly crystalline polymers opened into apertures when the film was either uniaxially or biaxially oriented, prior art thinking was that such thinned regions formed in the aforementioned softer, low-crystallinity polymers would not (and in fact did not), with any degree of predictability, open up into an apertured fabric upon uniaxial or biaxial orientation.
In fact, prior to this invention applicants believed that in order to form apertured webs from soft, low density, very low density and ultra low density polyethylene or other low crystallinity polymers, the molten, extruded film needed to be embossed by opposed patterned rolls (i.e., two-sided embossing) of the type disclosed in the Hovis et al. '794 patent. As indicated earlier, although the use of opposed patterned rolls does successfully form a defined apertured pattern in low density and very low density polyethylene film the three-dimensional, strand-like configuration imparted to the film has been perceived to be undesirable for a number of applications, including use as a facing sheet for sanitary absorbent products, such as sanitary napkins and incontinent pads.